Travelling grate combustion of coking and caking bituminous coals



July 14, 1959 c, ps 2,894,465

TRAVELLING GRATE COMBUSTION OF COKING AND CAKING BITUMINOUS COALS FiledJune 8, 1956 canzz zlz Em United States Patent TRAVEL'LHIG COMBUSTION OFC'OKING ANLD CAKING BITUMINOUS COALS CarriugtonfB. .Phillips,PennValley, Pa. Application June 8, 1956, Serial No. 590,328

'1 Claim. or. 110-69) The present invention relates to methods andfurnaces for burning coking and/or caking bituminous coals on travellinggrate stokers. Most bituminous coals coke or cake at least slightly andmany are strongly coking and/ or caking, and their burning on travellinggrate stokers ranges accordingly from troublesome to completelyimpractical.

- When reference is made herein to travelling grate stokers it isintended to refer to stokers of the character which progress a thin fuelbed forward while maintaining the fuel bed quiescent. These are knownvariously as chain grate, travelling grate, oscillating grate, vibratinggrate or moving grate stokers.

The purpose of the invention is to permit efficient and trouble-freeburning of coking and/or caking coals on travelling grate stokers bypreoxidizing the surface of the coal particles before combustion starts.

In the drawings I have chosen to illustrate one only of the numerousembodiments in which my invention may appear, selecting the form shownfrom the standpoints of convenience in illustration, satisfactoryoperation and clear demonstration of the principles involved.

Figure 1 is a central vertical longitudinal section of a travellinggrate furnace in accordance with the invention.

Figure 2 is a section of Figure 1 on the line 22.

Describing in illustration but not in limitation and referring to thedrawings.

Travelling grate stokers are, in many ways, superior to other types ofstokers but must have a uniformly burning fuel bed. When coal cokes orcakes on travelling grate stokers, the resistance to the flow ofcombustion air through the fuel bed becomes irregular, the combustion isinefiicient and the grates overheat because they are not protected by auniform and compact bed of ash. Nevertheless travelling grate stokersare very desirable as they avoid all of the following difficulties, oneor more of which are encountered in other stokers:

(a) Fly ash which requires expensive collectors and which results inoperation complications.

(b) Excessive manual attention.

() Troublesome clinkers.

((1) Not readily adaptable to automatic operation.

(e) Not readily adaptable to low-cost, shop-assembled boiler units.

(f) High maintenance costs.

(g) Not suitable for many of the smaller sizes of boiler units.

The present invention obtains a uniformly burning fuel bed on travellinggrate stokers with coking and/ or caking coals by preoxidizing thesurface of the coal particles before furnace heat causes stickingtogether of the coal particles and after the coal bed has becomequiescent.

It has been demonstrated that when preoxidizing air is passed throughcoal at a temperature of 200 to 500 degrees F. and the coal, therefore,is undisturbed before burning, all coking or caking will be eliminated.

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It has also been demonstrated that preoxidizing of coal will noteliminate its coking or caking during combustion, if the coal isdisturbed after preoxidizing and before com bustion.

The present invention provides. an arrangement for travelling gratestokers whereby preoxidation can take place within the limitationsdescribed in the preceding two paragraphs.

The invention provides, in connection with travelling grate stokers, aseparate fan connected to a compartment or zone, for preoxidizing air ata temperature of 200 to 500 degrees F., preferably between 200. and 400degrees F., immediately behind the stoker hopper and gate and extendingunder and across the entire width of the grate and by providing an archor other construction to shield the preoxidizing zone from the furnaceheat.

In accordance with the invention coal 20, which may be coking and/ orcaking bituminous coal in slack or plus mesh sizes, is fed from a hopper21 under a gate 22 to establish a coal bed 23. on a. travelling grate 24progressing in the direction. of the arrow into the combustion chamberof a coal furnace. The coal bed as well known in the art will have athickness of the order of 3 to 6 inches.

Immediately beyond the gate 22 the travelling grate carries the fuel bedabove an air chamber 25' extending from side to side under thetravelling grate, which receives air for preoxidation preheated in anysuitable way to a temperature of 200 to 500 degrees F., preferably 200to 400 degrees F. It will be understood that any desired preheatingmeans can be used, whether a separate preheater, heated by furnace gasor by a separate source of heat, or a combined preheater and blower, aswell known. 1 illustrate in Figure 2 a blower 26 supplying preheated airto the chamber 25. This may, if desired, be a preheating blower, adiagrammatic illustration of a preheater being shown at 26'.

The air passes up through the fuel bed at 27, encountering the fuel bedbefore the coal becomes plastic and has a tendency to stick together andwhile the coal particles are entirely separate and are thus all exposedto this air. To protect against too rapid heating of the fuel bed wherepreoxidizing is occurring, immediately beyond the gate 22 I provide afurnace wall or arch 28 whose lower side 30 hangs above the fuel bed andforms a protecting or shading arch. The arch is so close to the top ofthe fuel bed that there is only room for excess preoxidizing air to flowbetween it and the fuel bed. The lower side 30 desirably slopes upwardlygradually toward the furnace at an angle of 5 to 10 degrees to thehorizontal.

The nose of the arch conveniently has a radius at 31.

In the preferred form of the invention the lower side of the shieldingarch will be between 2 and 6 inches above the top of the fuel bed for adistance of 15 to 30 inches longitudinally of the stoker.

The excess preoxidizing air is deflected by the lower side of the archat 32 to make it flow into the combustion chamber and mix with thefurnace gases.

It will be understood that at a position beyond the preoxidizing zonethe fuel bed receives the main combustion air which passes suitablyupwardly at 33 under the action of a blower or the draft from a chimney,as well known.

In accordance with the invention therefore it is possi' ble to burncoking and/or caking bituminous coals on a travelling grate in anydesired coal size without additional precautions, as long as thepreoxidizing is carried out as here described. Thus all of theadvantages inherent in travelling grate stokers are made available inthe case of coking and/or caking bituminous coals. For example, the morelaborless operation of the stoker is made possible Fly ash is notproduced and fly ash collectors are not-required. Greater simplicity inoperation results. The devices of the invention are adaptable for smallboiler units and for package units.

Itwill be evident of course that the device of the invention is notlimited to coking and/ or caking bituminous coals but can be usedinterchangeably where required for burning free-burning bituminous coalsand also for burning lignite and sub-bituminous coals.

' In view of my invention and disclosure variations and modifications tomeet individual whim or particular need will doubtless become evident toothers skilled in the art, to obtainall or part of the benefits of myinvention without copying the method and apparatus shown, and I,therefore, claim all such insofar as they fall within the reasonablespirit and scope ofmy claim.

Havingthus described my invention what I claim as new and desire tosecure by Letters Patent is:

In a furnace for burning coking and/or caking bituminous coals, atravelling grate adapted to transport a fuel bed in quiescent condition,a hopper supplying coal to the grate, means for preheating air to atemperature between 200 and 500 degrees F., means for introducing thepreheated air beneath the travelling grate to pass through the fuel bedimmediately after the fuel bed is fed from the hopper and before thefuel bed enters distillation and combustion zones of the furnace,

an arch extending over the fuel bed for a length of from 15 to incheslongitudinally of the travelling grate and located between 2 and 6inches above the top of the fuel bed, said arch extending above theposition at which the preheated air is introduced, the lower side ofsaid arch sloping upwardly toward the furnace at an angle of 5 to 10degrees, said arch shielding the fuel bed from the intense furnace heat,whereby the fuel bed derives all of its heat immediately after it leavesthe hopper from the preheated air and is preoxidized so that it will notcoke or cake in distillation and combustion zones which follow, and anyexcess of preoxidizing air will flow into and mix with the furnacegases.

References Cited in the file of this patent UNITED STATES PATENTS1,450,127 Witz Mar. 27, 1923 1,834,261 Webster Dec. 1, 1931 1,839,741Davies Jan. 5,,1932

2,005,264 McNamara June 18, 1935 FOREIGN PATENTS 302,471 Great BritainDec. 20, 1928 OTHER REFERENCES Fuels and Their Combustion, by Haslam andRussell, pages 422 to 424, 1926 edition.

